A thermoelectric conversion material is an energy material which directly converts thermal energy and electrical energy based on two basic thermoelectric effects: the Seebeck effect and the Peltier effect.
As indicators for evaluation of the performance of a thermoelectric conversion material, a power factor P=S2σ and dimensionless figure of merit ZT=(S2σ/κ)T are used. Here, S: Seebeck coefficient, σ: conductivity, κ: thermal conductivity, T: absolute temperature. That is, to obtain a good thermoelectric characteristic, it is necessary that the Seebeck coefficient S and the conductivity a be high and the thermal conductivity κ be low.
To reduce the thermal conductivity κ, making the phonons, one of the factors in thermal conductivity, scatter is effective. A nanocomposite thermoelectric conversion material comprised of a thermoelectric conversion material matrix in which nanosize phonon scattering particles (PSP) are dispersed has been proposed.
As such a method of production of a nanocomposite thermoelectric conversion material, Japanese Patent Publication (A) No. 2008-147625 discloses a nanocomposite thermoelectric conversion material comprised of silica nanoparticles or other core particles on the surface of which Bi2(Te,Se)3 or another thermoelectric conversion material is grown as a shell to thereby obtain core/shell particles, wherein the core/shell particles are combined with each other to form a continuous phase constituted by a thermoelectric conversion material matrix by the shell and wherein, in the matrix, the core particles are thereby dispersed as phonon scattering particles.
Here, the core/shell particles comprised of the phonon scattering particles (for example silica) as the cores and the thermoelectric conversion material (for example Bi2(Te,Se)3) as the shells are prepared as follows:
As shown in FIG. 1, (1) a surface U of a silica particle P is modified, (2) Bi3+ cations are bonded with the functional groups of this modifier Q to form a complex, and (3) the Bi3+ cations of this complex are bonded with the Te2− anions and Se2− anions to (4) form Bi2(Te,Se)3 particles on the surface U. Along with the progress in formation of Bi2(Te,Se)3 on the surface U, as shown in FIG. 1(5), the phonon scattering particle P is surrounded by the Bi2(Te,Se)3 thermoelectric conversion material. By combining a large number of phonon scattering particles P in this state, as shown in FIG. 1(6), a nanocomposite thermoelectric conversion material comprised of a Bi2(Te,Se)3 thermoelectric conversion material as a matrix in which phonon scattering particles P are dispersed to a high degree is obtained.
This conventional method features the modification of (imparting functional groups to) the surface of the phonon scattering particles, formation of a Bi complex with the functional groups of this modifier, and reaction with the anions of Te and Se.
If making the Bi3+ cations directly react with the Te2− anions and selenium2− anions without modifying (imparting functional groups to) the surface U of the phonon scattering particles P, precipitates end up being rapidly formed in the reaction solution aside from the phonon scattering particle surface U, so phonon scattering particles P cannot be sufficiently taken into the precipitates formed, the excess phonon scattering particles P end up coagulating with each other, and a high dispersion state cannot be obtained.
In the conventional method, the phonon scattering particle surface U is modified, Bi complexes are formed, and Te and Se anions are reacted with this, whereby precipitates of Bi2(Te,Se)3 are preferentially formed at the surface U of the phonon scattering particles P. Furthermore, by using a reaction involving Bi complexes, since the reaction speed of Te2− anions and Se2− anions is slower compared with Bi3+ cations alone, rapid formation of precipitates at locations aside from the phonon scattering particle surface can be prevented.
According to this conventional method, it is possible to chemically cause the formation of Bi2(Te,Se)3 precipitates at the surface U of the phonon scattering particles P, so it is possible to obtain a nanocomposite thermoelectric conversion material which comprises a thermoelectric conversion material matrix in which nanosize phonon scattering particles are included in a highly dispersed state.
However, the nanocomposite thermoelectric conversion material obtained by this method, as shown in FIGS. 1(5) and (6), has organic phases derived from the modifier remaining at the surfaces of the cores, that is, phonon scattering particles. These not only lower the phonon scattering effect due to the phonon scattering particles, but also lower the conductivity of the nanocomposite thermoelectric conversion material, so result in the defect of the thermoelectric conversion performance dropping.